You were probably just fatigued from the pain meds the night before- just not on your game today.
When I bonk- it's quite obvious to me. It'll vary for everyone, but for me I get very irritable and often get teary/weepy for no apparent reason. When I start feeling it- it's too late to do anything about it and I usually finish the ride/run and call it quits for the day.
It took me the better part of last summer to figure out why sometimes I'd get so cranky during a long ride or long run (then it was like- duh!).